Solid-stage image pickup device and method for producing the same

ABSTRACT

A solid-state image pickup device includes an element isolation insulating film electrically isolating pixels on the surface of a well region; a first isolation diffusion layer electrically isolating the pixels under the element isolation insulating film; and a second isolation diffusion layer electrically isolating the pixels under the first isolation diffusion layer, wherein a charge accumulation region is disposed in the well region surrounded by the first and second isolation diffusion layers, the inner peripheral part of the first isolation diffusion layer forms a projecting region, an impurity having a conductivity type of the first isolation diffusion layer and an impurity having a conductivity type of the charge accumulation region are mixed in the projecting region, and a part of the charge accumulation region between the charge accumulation region and the second isolation diffusion layer is abutted or close to the second isolation diffusion layer under the projecting region.

CROSS REFERENCES TO RELATED APPLICATIONS

The subject matter of U. S. application Ser. No. 11/340,180, isincorporated herein by reference. The present application is aDivisional application of U.S. Ser. No. 11/340,180, filed Jan. 26, 2006,now U.S. Pat. No. 7,217,961 which claims priority to

Japanese Patent Application JP 2005-024761 filed in the Japanese PatentOffice on Feb. 1, 2005, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image pickup device thatcan improve the photoelectric conversion efficiency of a unit pixel anda method for producing the same.

2. Description of the Related Art

A complementary metal oxide semiconductor (CMOS) type solid-state imagepickup device, which is one type of image sensor, includes a pixel areaand peripheral circuits that drive the pixel area and process signals.Miniaturization technologies for the peripheral circuits have beengenerally applied to the pixel area. In addition, in theelement-isolation of a pixel of the recent fine CMOS type solid-stateimage pickup device, a method of shallow trench isolation (STI) has beenused as in the case of the peripheral circuits.

FIG. 7 is a cross-sectional view showing a schematic structure of aknown general CMOS image sensor.

In FIG. 7, the CMOS image sensor includes an n-type siliconsemiconductor substrate 1; a p-type semiconductor well region 2 disposedon the n-type silicon semiconductor substrate 1; a p⁺-type holeaccumulation layer 3, an n-type signal charge accumulation region 4, areading region 11, an n⁺-type reading part (FD) 12, and a trench 5 forshallow trench isolation (STI), which are disposed in the p-typesemiconductor well region 2. An insulating material 6 such as siliconoxide is embedded in the trench 5 and a P⁺-type STI side wall layer 7 isdisposed outside of the insulating material 6. A reading gate 9 isdisposed above the reading region 11 with an insulating film 8 disposedtherebetween. A reset gate 10 is disposed adjacent to the reading gate9. A light-receiving part of such a CMOS image sensor is composed of anembedded photodiode including the hole accumulation layer 3 provided onthe top face of the substrate and the signal charge accumulation region4 provided under the hole accumulation layer 3 (refer to JapaneseUnexamined Patent Application Publication No. 2002-231929).

SUMMARY OF THE INVENTION

In the CMOS image sensor shown in FIG. 7, from the viewpoint ofsuppressing the generation of dark current, the STI side wall layer 7has at least a certain width (thickness), in other words, has at least acertain concentration. However, when the STI side wall layer 7 has acertain width or more, the effective area of the signal chargeaccumulation region 4 of the photodiode is decreased, resulting in adecrease in the amount of electric charge to be handled. Consequently,in order to increase the amount of electric charge, i.e., the amount ofsaturation signal (the amount of signal charge) handled by thephotodiode per unit area, a method of increasing not only the impurityconcentration of the n⁺ layer of the signal charge accumulation region 4but also the impurity concentration of the surface p⁺ layer and that ofthe p layer of the well region 2 disposed under the signal chargeaccumulation region 4 is employed.

However, in the solid-state image pickup device having such a structure,the electric field between the n⁺ layer of the signal chargeaccumulation region 4 and the surface p⁺ layer easily increases. Theincrease in the electric field disadvantageously increases dark currentand white spots. Also, in order to increase the amount of electriccharge handled by the photodiode per unit area, when the impurityconcentration of the n⁺ layer of the signal charge accumulation region 4is increased, the electric field with the hole accumulation layer 3 isincreased, thereby easily generating white spots.

The present invention has been made in view of these situations, and itis desirable to provide a solid-state image pickup device in which thephotoelectric conversion efficiency of a unit pixel can be improved andcharacteristics relating to problems such as white spots can be improvedand a method for producing the same.

According to an embodiment of the present invention, there is provided asolid-state image pickup device in which a plurality of pixels includinga photoelectric conversion part having a charge accumulation regionaccumulating signal charges is two-dimensionally arrayed in a wellregion provided on a semiconductor substrate, the solid-state imagepickup device including an element isolation insulating film thatelectrically isolates the pixels on the surface of the well region; afirst isolation diffusion layer that electrically isolates the pixelsunder the element isolation insulating film; and a second isolationdiffusion layer that electrically isolates the pixels under the firstisolation diffusion layer, wherein the charge accumulation region isdisposed in the well region surrounded by the first isolation diffusionlayer and the second isolation diffusion layer, the inner peripheralpart of the first isolation diffusion layer corresponding to theboundary between the charge accumulation region and the first isolationdiffusion layer forms a projecting region projecting in the chargeaccumulation region, an impurity having a conductivity type of the firstisolation diffusion layer and an impurity having a conductivity type ofthe charge accumulation region are mixed in the projecting region, and apart of the charge accumulation region corresponding to the boundarybetween the charge accumulation region and the second isolationdiffusion layer is disposed under the projecting region and is abuttedor disposed close to the second isolation diffusion layer.

According to an embodiment of the present invention, there is provided amethod for producing a solid-state image pickup device in which aplurality of pixels including a photoelectric conversion part having acharge accumulation region accumulating signal charges istwo-dimensionally arrayed in a well region formed on a semiconductorsubstrate, the method including the steps of forming an elementisolation insulating film on the surface of the well region, the elementisolation insulating film electrically isolating the pixels; forming afirst isolation diffusion layer in the well region, the first isolationdiffusion layer electrically isolating the pixels under the elementisolation insulating film and having a projecting region in which theinner peripheral part of the first isolation diffusion layercorresponding to the boundary with the charge accumulation regionprojects in the charge accumulation region; forming a second isolationdiffusion layer in the well region, the second isolation diffusion layerelectrically isolating the pixels under the first isolation diffusionlayer; and forming the photoelectric conversion part for each pixel inthe well region in which the photoelectric conversion part iselectrically isolated from each other by the element isolationinsulating film, the first isolation diffusion layer, and the secondisolation diffusion layer, wherein the step of forming the photoelectricconversion part includes the steps of implanting an impurity ion forforming the charge accumulation region in the well region surrounded bythe first isolation diffusion layer and the second isolation diffusionlayer and the projecting region of the first isolation diffusion layer;and thermally diffusing the impurity implanted in the well region by thestep of ion implantation so that a part of the charge accumulationregion corresponding to the boundary between the charge accumulationregion and the second isolation diffusion layer is disposed under theprojecting region and is abutted or disposed close to the secondisolation diffusion layer.

According to an embodiment of the present invention, there is provided amethod for producing a solid-state image pickup device in which aplurality of pixels including a photoelectric conversion part having acharge accumulation region accumulating signal charges istwo-dimensionally arrayed in a well region formed on a semiconductorsubstrate, the method including the steps of forming an elementisolation insulating film on the surface of the well region, the elementisolation insulating film electrically isolating the pixels; forming afirst isolation diffusion layer in the well region, the first isolationdiffusion layer electrically isolating the pixels under the elementisolation insulating film and having a projecting region in which theinner peripheral part of the first isolation diffusion layercorresponding to the boundary with the charge accumulation regionprojects in the charge accumulation region; forming a second isolationdiffusion layer in the well region, the second isolation diffusion layerelectrically isolating the pixels under the first isolation diffusionlayer; and forming the photoelectric conversion part for each pixel inthe well region in which the photoelectric conversion part iselectrically isolated from each other by the element isolationinsulating film, the first isolation diffusion layer, and the secondisolation diffusion layer, wherein the step of forming the photoelectricconversion part includes the steps of implanting an impurity ion forforming the charge accumulation region in the well region surrounded bythe first isolation diffusion layer and the second isolation diffusionlayer; and thermally diffusing the impurity implanted in the well regionby the step of ion implantation so that a part of the chargeaccumulation region corresponding to the boundary between the chargeaccumulation region and the second isolation diffusion layer is disposedunder the projecting region and is abutted or disposed close to thesecond isolation diffusion layer, and the projecting region of the firstisolation diffusion layer is formed in a state in which an impurityhaving the conductivity type of the first isolation diffusion layer andan impurity having a conductivity type of the charge accumulation regionare mixed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the structure of the relevant part in a processfor producing a solid-state image pickup device according to a firstembodiment of the present invention;

FIG. 2 is a view showing the structure of the relevant part in theprocess for producing the solid-state image pickup device according tothe first embodiment of the present invention;

FIG. 3 is a view showing the structure of the relevant part in theprocess for producing the solid-state image pickup device according tothe first embodiment of the present invention;

FIG. 4 is a view showing the structure of the relevant part in a processfor producing a solid-state image pickup device according to a secondembodiment of the present invention;

FIG. 5 is a view showing the structure of the relevant part in theprocess for producing the solid-state image pickup device according tothe second embodiment of the present invention;

FIG. 6 is a view showing the structure of the relevant part in theprocess for producing the solid-state image pickup device according tothe second embodiment of the present invention; and

FIG. 7 is a cross-sectional view showing the schematic structure of aknown general CMOS image sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a solid-state image pickup device of an embodiment of thepresent invention, the inner peripheral part of a first isolationdiffusion layer corresponding to the boundary between a chargeaccumulation region and the first isolation diffusion layer forms aprojecting region projecting in the charge accumulation region, a partof the charge accumulation region corresponding to the boundary betweenthe charge accumulation region of a photoelectric conversion part and asecond isolation diffusion layer is disposed under the projecting regionand is abutted or disposed close to the second isolation diffusionlayer, and the projecting region is formed in a state in which animpurity having a conductivity type of the first isolation diffusionlayer and an impurity having a conductivity type of the chargeaccumulation region are mixed.

This structure can increase the charge accumulation region (area) of thephotoelectric conversion part in a pixel to improve the photoelectricconversion efficiency of a unit pixel.

First Embodiment

A solid-state image pickup device according to an embodiment of thepresent invention and a method for producing the same will now bedescribed with reference to the drawings.

FIGS. 1 to 3 are views showing the structure of the relevant part in aprocess for producing a solid-state image pickup device according to afirst embodiment of the present invention.

A solid-state image pickup device 20 according to the first embodimentincludes a first conductivity type semiconductor substrate, for example,an n-type silicon substrate 21. A second conductivity type, for example,a p-type semiconductor well region 22 is disposed on the n-type siliconsubstrate 21. In the p-type semiconductor well region 22, a plurality ofpixels 30 each including a photodiode PD serving as a photoelectricconversion part (the pixels 30 including transistors (not shown in thefigures) for taking out signal charges accumulated in the photodiodesPDs) is disposed in a two-dimensional array. An element isolationinsulating film 23 for electrically isolating adjacent unit pixels 30from each other is disposed so as to face the surface of the p-typesemiconductor well region 22. Furthermore, a p-type first isolationdiffusion layer 24 for electrically isolating the adjacent unit pixels30 from each other is disposed on the surface of the p-typesemiconductor well region 22, the surface being disposed under theelement isolation insulating film 23. Furthermore, a p-type secondisolation diffusion layer 25 for electrically isolating the adjacentunit pixels 30 from each other is disposed under the first isolationdiffusion layer 24 in the p-type semiconductor well region 22. Aninsulating film 26 is provided on the surface of the p-typesemiconductor well region 22.

The photodiode PD is disposed in the p-type semiconductor well region 22surrounded by the p-type semiconductor well region 22, the firstisolation diffusion layers 24, and the second isolation diffusion layers25 and includes a first conductivity type n-type charge accumulationregion 27 that accumulates signal charges generated by photoelectricconversion. In the present embodiment, the photodiode PD furtherincludes a second conductivity type p⁺ accumulation layer 28 disposed onthe surface of the n-type charge accumulation region 27. Such aphotodiode PD forms an image sensor having a hole accumulation diode(HAD) structure.

In the photodiode PD, the p⁺ accumulation layer 28 has a function ofsuppressing the cause of dark current and white spots due to theinterface level. The first isolation diffusion layer 24 isolatesadjacent photodiodes PDs more reliably. In addition, the secondisolation diffusion layer 25 doubles as a function for isolating theadjacent photodiodes PDs in the depth direction.

A method for producing the solid-state image pickup device 20 will nowbe described with reference to FIGS. 1 to 3.

First, as shown in FIG. 1, the p-type semiconductor well region 22 isformed on the n-type silicon substrate 21. Subsequently, an isolationpattern for isolating the pixels 30 including the photodiodes PDsserving as the photoelectric conversion part from each other ispatterned on the p-type semiconductor well region 22 byphotolithography. The p-type second isolation diffusion layer 25 is thenformed in the depth direction by implanting an impurity ion once ormultiple times. In this case, the dosage during ion implantation isabout 1×10¹¹ to 1×10¹⁴ cm⁻². In addition, the second isolation diffusionlayer 25 has a width d1 of about 0.05 to 10 μm. Thus, the width d1 ofthe second isolation diffusion layer 25 is set to the above dimension,and in addition, the line width d1 is decreased as much as possible, aslong as the process can be performed. Thus, the width d1 is controlledto be smaller than the line width dimension d2 of the element isolationinsulating film 23. Consequently, an extension region 29 forsubstantially extending the charge accumulation area of the n-typecharge accumulation region 27 is formed in an area of the p-typesemiconductor well region 22, the area being disposed under the elementisolation insulating film 23 and inside the second isolation diffusionlayer 25.

Subsequently, an element-isolation pattern for isolating the pixels 30including the photodiodes PDs serving as the photoelectric conversionpart from each other is patterned on the surface of the p-typesemiconductor well region 22 by photolithography. The p-type firstisolation diffusion layer 24 is then formed by implanting an impurityion directly on the second isolation diffusion layer 25. In this case,the dosage during ion implantation is about 1×10¹¹ to 1×10¹⁴ cm⁻². Inaddition, the first isolation diffusion layer 24 is formed so that thewidth in the horizontal direction of the first isolation diffusion layer24 is substantially the same as the line width d2 of the elementisolation insulating film 23. Consequently, as shown in FIG. 2, theinner peripheral part of the first isolation diffusion layer 24corresponding to the boundary between the charge accumulation region 27and the first isolation diffusion layer 24 is projected in the chargeaccumulation region 27 to form a projecting region 24A.

Furthermore, an element-isolation pattern for isolating the pixels 30including the photodiodes PDs from each other is patterned on thesurface of the p-type semiconductor well region 22 by photolithography.The element isolation insulating film 23 composed of, for example,silicon oxide (SiO₂) is then formed on the first isolation diffusionlayer 24 so as to overlap with each other. Subsequently, in the p-typesemiconductor well region 22 surrounded by the first isolation diffusionlayer 24 and the second isolation diffusion layer 25, and the projectingregions 24A of the first isolation diffusion layer 24, an impurity ionis implanted in a dosage of, for example, about 1×10¹² to 1×10¹⁴ cm⁻²through an opening 23A formed in the element isolation insulating film23 and the element isolation insulating film 23. Consequently, then-type charge accumulation region 27 having an area corresponding to theopening 23A is formed, and at the same time, the projecting region 24Aof the first isolation diffusion layer 24 becomes a state in which theimpurity having a conductivity type of the first isolation diffusionlayer 24, for example, a p-type impurity, and the impurity having aconductivity type of the charge accumulation region 27, for example, ann-type impurity are mixed. By forming the state in which the impuritiesare mixed in the projecting region 24A, the electric field of thejunction between the n layer and the p layer in the vicinity of thecharge accumulation region 27 is reduced. Subsequently, a p-typeimpurity ion with a high concentration of, for example, 5×10¹⁷ cm³ ormore is implanted on the surface of the n-type charge accumulationregion 27 and diffused to form the p⁺ accumulation layer 28.

Subsequently, the solid-state image pickup device 20 having thestructure shown in FIG. 2 is charged in a thermal diffusion furnace (notshown in the figure) and heated at a predetermined temperature, forexample, under an atmosphere of 1,000° C. for a predetermined time, forexample, 10 seconds. Thereby, the impurity of the n-type chargeaccumulation region 27 is thermally diffused in the p-type semiconductorwell region 22. Consequently, as shown in FIG. 3, the n-type chargeaccumulation region 27 is extended in the depth direction of thephotodiode PD and in the direction orthogonal to the depth direction, atleast in the direction orthogonal to the depth direction of thephotodiode PD. Thus, the n-type charge accumulation region 27 is formedso that the peripheral part of the n-type charge accumulation region 27is abutted or disposed close to the second isolation diffusion layer 25under the projecting region 24A. Here, the term “close to” means thatthe n-type charge accumulation region 27 extends toward the p-typesecond isolation diffusion layer 25 by at least half of the width of theextension region 29.

According to the solid-state image pickup device 20 of the firstembodiment and the method for producing the same, in the step of formingthe photodiode PD serving as the photoelectric conversion part, thecharge accumulation region 27 of the photodiode PD is formed in thep-type semiconductor well region 22 surrounded by the first isolationdiffusion layer 24 and the second isolation diffusion layer 25 by ionimplantation, and in addition, the impurity ion is implanted in theprojecting region 24A of the first isolation diffusion layer 24. By thesubsequent thermal diffusion, the peripheral part of the n-type chargeaccumulation region 27 is abutted or disposed close to the secondisolation diffusion layer 25 under the projecting region 24A. Therefore,the charge accumulation region 27 can be easily formed under theprojecting region 24A of the first isolation diffusion layer 24. Thisstructure can increase the charge accumulation region (area) of thephotoelectric conversion part in the pixel to improve the photoelectricconversion efficiency of the unit pixel and increase the amount ofsaturation signal. Furthermore, unlike the known photoelectrictransducer, the impurity concentration in the impurity region of aphotoelectric transducer is not excessively increased, and therefore,the generation of defective pixels such as white spots can besuppressed. In addition, the projecting region 24A is formed in a statein which the impurity having the conductivity type of the firstisolation diffusion layer 24 and the impurity having the conductivitytype of the charge accumulation region 27 are mixed. Consequently, theelectric field of the junction between the n layer in the chargeaccumulation region 27 and the p layer can be reduced. Thereby, thegeneration of electrons is suppressed, and thus the generation of darkcurrent and white spots can be suppressed.

Second Embodiment

A solid-state image pickup device according to a second embodiment ofthe present invention and a method for producing the same will now bedescribed with reference to FIGS. 4 to 6.

FIGS. 4 to 6 are views showing the structure of the relevant part in aprocess for producing the solid-state image pickup device according tothe second embodiment of the present invention.

As in the case shown in Example 1, a solid-state image pickup device 40according to the second embodiment also includes a first conductivitytype semiconductor substrate, for example, an n-type silicon substrate21. A second conductivity type, for example, a p-type semiconductor wellregion 22 is disposed on the n-type silicon substrate 21. In the p-typesemiconductor well region 22, a plurality of pixels 30 each including aphotodiode PD serving as a photoelectric conversion part is disposed ina two-dimensional array. An element isolation insulating film 23 forelectrically isolating adjacent unit pixels 30 from each other isdisposed so as to face the surface of the p-type semiconductor wellregion 22. Furthermore, a p-type first isolation diffusion layer 24 forelectrically isolating the adjacent unit pixels 30 from each other isdisposed on the surface of the p-type semiconductor well region 22, thesurface being disposed under the element isolation insulating film 23.Furthermore, a p-type second isolation diffusion layer 25 forelectrically isolating the adjacent unit pixels 30 from each other isdisposed under the first isolation diffusion layer 24 in the p-typesemiconductor well region 22. An insulating film 26 is provided on thesurface of the p-type semiconductor well region 22.

The photodiode PD is disposed in the p-type semiconductor well region 22surrounded by the p-type semiconductor well region 22, the firstisolation diffusion layers 24, and the second isolation diffusion layers25 and includes a first conductivity type n-type charge accumulationregion 27 that accumulates signal charges generated by photoelectricconversion. The photodiode PD further includes a second conductivitytype p⁺ accumulation layer 28 disposed on the surface of the n-typecharge accumulation region 27. Such a photodiode PD forms an imagesensor having the HAD structure.

A method for producing the solid-state image pickup device 40 will nowbe described with reference to FIGS. 4 to 6.

First, as shown in FIG. 4, the p-type semiconductor well region 22 isformed on the n-type silicon substrate 21. Subsequently, an isolationpattern for isolating the pixels 30 including the photodiodes PDsserving as the photoelectric conversion part from each other ispatterned on the p-type semiconductor well region 22 byphotolithography. The p-type second isolation diffusion layer 25 is thenformed in the depth direction by implanting an impurity ion once ormultiple times. In this case, the dosage during ion implantation isabout 1×10¹¹ to 1×10¹⁴ cm⁻². The width d1 of the second isolationdiffusion layer 25 is controlled to be smaller than the line widthdimension d2 of the element isolation insulating film 23. Consequently,an extension region 29 for substantially extending the chargeaccumulation area of the n-type charge accumulation region 27 is formedin an area of the p-type semiconductor well region 22, the area beingdisposed under the first isolation diffusion layer 24 and inside thesecond isolation diffusion layer 25.

Subsequently, an element-isolation pattern for isolating the pixels 30including the photodiodes PDs serving as the photoelectric conversionpart from each other is patterned on the surface of the p-typesemiconductor well region 22 by photolithography. The p-type firstisolation diffusion layer 24 is then formed by implanting an impurityion directly on the second isolation diffusion layer 25. In this case,the dosage during ion implantation is about 1×10¹¹ to 1×10¹⁴ cm⁻². Inaddition, the first isolation diffusion layer 24 is formed so that thewidth in the horizontal direction of the first isolation diffusion layer24 is substantially the same as the line width d2 of the elementisolation insulating film 23. Consequently, as shown in FIG. 5, theinner peripheral part of the first isolation diffusion layer 24corresponding to the boundary between the charge accumulation region 27and the first isolation diffusion layer 24 forms a projecting region 24Aprojecting in the charge accumulation region 27 by a predetermineddimension.

Furthermore, an element-isolation pattern for isolating the pixels 30including the photodiodes PDs from each other is patterned on thesurface of the p-type semiconductor well region 22 by photolithography.The element isolation insulating film 23 composed of, for example,silicon oxide (SiO₂) is then formed on the first isolation diffusionlayer 24 so as to overlap with each other. Subsequently, in the p-typesemiconductor well region 22 surrounded by the first isolation diffusionlayer 24 and the second isolation diffusion layer 25, an impurity ion isimplanted in a dosage of, for example, about 1×10¹² to 1×10¹⁴ cm⁻²through an opening 23A formed in the element isolation insulating film23. Consequently, the n-type charge accumulation region 27 having anarea corresponding to the opening 23A is formed. Subsequently, a p-typeimpurity ion with a high concentration of, for example, 5×10¹⁷ cm³ ormore is implanted on the surface of the n-type charge accumulationregion 27 and diffused to form the p⁺ accumulation layer 28.

Subsequently, the solid-state image pickup device 40 having thestructure shown in FIG. 5 is charged in a thermal diffusion furnace (notshown in the figure) and heated at a predetermined temperature, forexample, under an atmosphere of 1,000° C. for a predetermined time, forexample, 10 seconds. Thereby, the impurity of the n-type chargeaccumulation region 27 is thermally diffused, i.e., subjected to rapidthermal annealing (RTA), in the p-type semiconductor well region 22.Consequently, as shown in FIG. 6, the n-type charge accumulation region27 is extended in the depth direction of the photodiode PD and in thedirection orthogonal to the depth direction, at least in the directionorthogonal to the depth direction of the photodiode PD. Thus, the n-typecharge accumulation region 27 is formed so that the peripheral part ofthe n-type charge accumulation region 27 is abutted or disposed close tothe second isolation diffusion layer 25 under the projecting region 24A.At the same time, the impurity implanted in the n-type chargeaccumulation region 27 by ion implantation is also thermally diffused inthe projecting region 24A of the first isolation diffusion layer 24.Consequently, the projecting region 24A becomes a state in which theimpurity having a conductivity type of the first isolation diffusionlayer 24, for example, a p-type impurity, and the impurity having aconductivity type of the charge accumulation region 27, for example, ann-type impurity are mixed. By forming the state in which the p-typeimpurity and the n-type impurity are mixed in the projecting region 24A,the electric field of the junction between the n layer and the p layerin the vicinity of the charge accumulation region 27 is reduced.

According to the solid-state image pickup device 40 of the secondembodiment and the method for producing the same, in the step of formingthe photodiode PD serving as the photoelectric conversion part, thecharge accumulation region 27 of the photodiode PD is formed in thep-type semiconductor well region 22 surrounded by the first isolationdiffusion layer 24 and the second isolation diffusion layer 25 byimplanting an impurity ion. By the subsequent thermal diffusion, theperipheral part of the n-type charge accumulation region 27 is abuttedor disposed close to the second isolation diffusion layer 25 under theprojecting region 24A. Therefore, the charge accumulation region 27 canbe easily formed under the projecting region 24A of the first isolationdiffusion layer 24. This structure can increase the charge accumulationregion (area) of the photoelectric conversion part in the pixel toimprove the photoelectric conversion efficiency of the unit pixel andincrease the amount of saturation signal. Furthermore, unlike the knownphotoelectric transducer, the impurity concentration in the impurityregion of photoelectric transducer is not excessively increased, andtherefore, the generation of defective pixels such as white spots can besuppressed. In addition, the impurity implanted in the n-type chargeaccumulation region 27 by ion implantation is also thermally diffused inthe projecting region 24A. Thus, the projecting region 24A is in a statein which the impurity having the conductivity type of the firstisolation diffusion layer 24 and the impurity having the conductivitytype of the charge accumulation region 27 are mixed. Consequently, theelectric field of the junction between the n layer in the chargeaccumulation region 27 and the p layer can be reduced. Thereby, thegeneration of electrons is suppressed, and thus the generation of darkcurrent and white spots can be suppressed.

The first embodiment and the second embodiment describe the cases wherethe photodiode PD forms an image sensor having the HAD structure inwhich the p⁺ accumulation layer 28 is laminated on the n-type chargeaccumulation region 27. However, the present invention is not limitedthereto and the photodiode PD may have a structure, for example, thatdoes not include the p⁺ accumulation layer 28.

The present invention is not limited to the above embodiments. In thespecific structures, functions, operations, and advantages, the presentinvention may be embodied by other various modifications withoutdeparting from the spirit and the scope thereof. In the aboveembodiments, examples in which the present invention is mainly appliedto a CMOS image sensor have been described. However, when the presentinvention is applied to other image pickup devices such as a CCD imagepickup device, the same advantages can be achieved.

1. A method for producing a solid-state image pickup device in which aplurality of pixels including a photoelectric conversion part having acharge accumulation region accumulating signal charges istwo-dimensionally arrayed in a well region formed on a semiconductorsubstrate, the method comprising the steps of: forming an elementisolation insulating film on the surface of the well region, the elementisolation insulating film electrically isolating the pixels; forming afirst isolation diffusion layer in the well region, the first isolationdiffusion layer electrically isolating the pixels under the elementisolation insulating film and having a projecting region in which theinner peripheral part of the first isolation diffusion layercorresponding to the boundary with the charge accumulation regionprojects in the charge accumulation region; forming a second isolationdiffusion layer in the well region, the second isolation diffusion layerelectrically isolating the pixels under the first isolation diffusionlayer; and forming the photoelectric conversion part for each pixel inthe well region in which the photoelectric conversion part iselectrically isolated from each other by the element isolationinsulating film, the first isolation diffusion layer, and the secondisolation diffusion layer, wherein the step of forming the photoelectricconversion part includes the steps of implanting an impurity ion forforming the charge accumulation region in the well region surrounded bythe first isolation diffusion layer and the second isolation diffusionlayer and the projecting region of the first isolation diffusion layer;and thermally diffusing the impurity implanted in the well region by thestep of ion implantation so that a part of the charge accumulationregion corresponding to the boundary between the charge accumulationregion and the second isolation diffusion layer is disposed under theprojecting region and is abutted or disposed close to the secondisolation diffusion layer.
 2. A method for producing a solid-state imagepickup device in which a plurality of pixels including a photoelectricconversion part having a charge accumulation region accumulating signalcharges is two-dimensionally arrayed in a well region formed on asemiconductor substrate, the method comprising the steps of: forming anelement isolation insulating film on the surface of the well region, theelement isolation insulating film electrically isolating the pixels;forming a first isolation diffusion layer in the well region, the firstisolation diffusion layer electrically isolating the pixels under theelement isolation insulating film and having a projecting region inwhich the inner peripheral part of the first isolation diffusion layercorresponding to the boundary with the charge accumulation regionprojects in the charge accumulation region; forming a second isolationdiffusion layer in the well region, the second isolation diffusion layerelectrically isolating the pixels under the first isolation diffusionlayer; and forming the photoelectric conversion part for each pixel inthe well region in which the photoelectric conversion part iselectrically isolated from each other by the element isolationinsulating film, the first isolation diffusion layer, and the secondisolation diffusion layer, wherein the step of forming the photoelectricconversion part includes the steps of implanting an impurity ion forforming the charge accumulation region in the well region surrounded bythe first isolation diffusion layer and the second isolation diffusionlayer; and thermally diffusing the impurity implanted in the well regionby the step of ion implantation so that a part of the chargeaccumulation region corresponding to the boundary between the chargeaccumulation region and the second isolation diffusion layer is disposedunder the projecting region and is abutted or disposed close to thesecond isolation diffusion layer, and the projecting region of the firstisolation diffusion layer is formed in a state in which an impurityhaving the conductivity type of the first isolation diffusion layer andan impurity having a conductivity type of the charge accumulation regionare mixed.
 3. The method for producing a solid-state image pickup deviceaccording to claim 1 or claim 2, wherein the second isolation diffusionlayer is formed so as to extend in the depth direction of the wellregion.
 4. The method for producing a solid-state image pickup deviceaccording to claim 1 or claim 2, wherein the photoelectric conversionpart comprises a hole accumulation layer laminated on the chargeaccumulation region.